The present invention relates to printed circuit boards and more particularly to high-frequency EMC containment therefor.
Electronic components, including semiconductor components, integrated circuits and chips, carried on printed circuit boards are a source of electromagnetic interference or radio frequency disturbances when in use. Manufacturers of electronic devices must provide means to contain such electromagnetic interference or radio frequency disturbances when the devices are in use (EMC). For ease, both electromagnetic interference and radio frequency disturbances will be referred to as electromagnetic emissions.
Coating or xe2x80x9cplatingxe2x80x9d a printed circuit board with a film of highly conductive material such as copper over all surfaces enables the printed circuit board to effectively operate electrically as a Faraday cage, and thereby constitute one means to contain such electromagnetic emissions.
To manufacture a conductively shielded, or xe2x80x9cplatedxe2x80x9d, printed circuit board, it is typical to provide the printed circuit board with extra material, or xe2x80x9cscrapxe2x80x9d, which provides a location at which to support the printed circuit board during the plating step. The scrap border is typically connected to the printed circuit board by support tabs. Following plating, the scrap border is removed leaving the final printed circuit board.
However, the removal of the scrap border results in a xe2x80x9cbreakxe2x80x9d in the conductive shielding where the support tabs were severed, leaving the final board susceptible to electromagnetic emissions entering or leaving the board at the site of these support tabs.
One solution previously employed was to drill through a printed circuit board alternating rows of shielding holes, or vias, inward of each support tab. The vias are interiorly coated or filled with the conductive material. The vias come into contact with all ground layers in the printed circuit board, thereby connecting the ground layers to the top and bottom conductive layers of the printed circuit board. These vias attenuate frequencies of electromagnetic waves along their interior lengths, thereby effectively reducing the break in the conductive shield at the support tabs, and thus further reduce the chance of electromagnetic waves travelling on the inside surface of the top or bottom conductive surface of the printed control board from reaching and travelling along the outside surface and contributing to radiated emissions.
However, the attenuation of high frequency electromagnetic waves is problematic. As frequencies increase, the diameter and spacing of the vias must be decreased to accomplish signal attenuation. It is difficult to produce vias fine enough, or produce a fine enough piton to produce the vias, to attenuate signal energies in excess of 622 megahertz having regard to such factors as the thickness of printed circuit boards and manufacturing limitations.
Accordingly it is desirable to have an alternative means to attenuate high frequency signal energies on plated printed circuit boards.
The present invention seeks to provide a printed circuit board which minimizes the above problems.
According to one aspect of the invention, there is provided a printed circuit board having a plurality of spaced apart scrap border support tabs along the perimeter of the board. The board surfaces are coated with a conductive shielding material, except that each tab presents an uncoated, unshielded surface at the point of severance created by detachment of the scrap border subsequent to coating application. The printed circuit board includes a plurality of spaced apart elongated apertures adjacent to the perimeter, with each aperture being inwardly coincident to a respective one of each of the support tabs, each aperture defining an inner surface adjacent to the corresponding support tab with a portion of the inner surface being substantially parallel to the perimeter of the circuit board, the inner surface of each aperture also being coated with the conductive shielding material with the latter being attached to the conductive shielding material of the board surface, such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
The invention defined above extends to all forms of multiple layer printed circuit boards including backplanes and panels used in electronic systems.
In another aspect of the invention there is a printed circuit assembly provided with a scrap border detachably connected to the perimeter of a printed circuit board by a plurality of spaced apart support tabs, all of which are coated with a conductive shielding material during manufacture. The scrap border is detached from the printed circuit board subsequent to coating by severance of the tabs whereby each tab presents an uncoated, unshielded surface at the point of severance. The printed circuit board includes a plurality of spaced apart elongated apertures adjacent to the perimeter of the board, with each aperture being inwardly coincident to a respective one of each of the support tabs. Each aperture defines an inner surface and an edge adjacent to the corresponding support tab, with a portion of the inner surface and the edge being substantially parallel to the perimeter of the circuit board, with the entire inner surface of each aperture also being coated with the conductive shielding material during manufacture, with the latter being attached to the conductive shielding material on the board surfaces such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
In yet another aspect of the invention there is provided a method of manufacturing a printed circuit assembly having multiple layers in a sandwich arrangement comprising: providing a circuit assembly including a scrap border extending around and detachably connected to the perimeter of a centrally disposed printed circuit board by a plurality of spaced apart support tabs, fully coating the assembly with a conductive shielding material, and detaching the scrap border from the printed circuit board subsequent to coating by severance of the tabs, whereby each tab presents an uncoated, unshielded surface at the point of severance; the method being characterized by the formation in the centrally located printed circuit board of a plurality of spaced apart elongated apertures adjacent the perimeter, with each aperture being inwardly of the perimeter coincident to a respective one of each of the support tabs, and formed such that each aperture defines an inner surface and an edge adjacent to the corresponding support tab, with a portion of the inner surface and the edge being substantially parallel to the perimeter of the circuit board, with the coating step being carried out such that the inner surface of each aperture becomes coated with the conductive shielding material and attached to the conductive shielding material on the board surface such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
Such an invention allows a plated printed control board to be EMC compliant at any operating frequency with signal energies up to and in excess of 40 gigahertz.
A separate enclosure around a printed circuit board carrying electronic components that can attenuate electromagnetic emissions may be used as another means to contain electromagnetic emissions. Such enclosures are typically referred to as Faraday cages.
Previously, Faraday cage enclosures surrounding a printed circuit board carrying electronic components on both major surfaces comprised two units which came into contact with each other. An electrically conductive gasket was applied on the edges of the two Faraday cage units to maintain electrical contact between the two units. However, it was difficult to maintain the gasket seal in circumstances where printed circuit boards comprising multiple layers were very thick.
An additional advantage of the present invention is that separate and independent Faraday shields which are EMC compliant, may be provided on each of the first (top) and second (bottom) major surfaces of a printed circuit board. These major surfaces carry the printed circuit board""s electronic components which generate electromagnetic emissions when in use.
In another alternative embodiment, there is provided a housing assembly for a printed circuit board having first and second opposing major surfaces and a perimeter surface. Each surface is coated with a conductive shielding material. The board further has one or more ground planes disposed with the ground planes being in electrical contact with the first and second major surfaces of the board. The board is further provided with electronic components carried on each of the first and second surfaces. The housing assembly comprises a first housing in overlying spaced relationship to the first major surface and in electrical contact thereto all around the board perimeter such that a first Faraday cage is formed by said first housing and said first major surface of the board, and a second housing in overlying spaced relationship with the second major surface and in electrical contact thereto, all around the board perimeter such that a second Faraday cage is formed by said second housing and said second major surface of the board.
Such an aspect may also be used to facilitate heat dissipation.
More particularly, high speed digital components carried on printed circuit boards are dissipating more power in higher speed applications. These components are generating proportionately more heat while in use. It is necessary to remove this heat so as to prevent overheating which could damage and ultimately result in breakdown of one or more electronic components, which are often costly or difficult to replace.
Heats sinks have previously been employed for heat dissipation from electrical components in use. The heat sink is provided on the same side of a printed circuit board as the electronic components, such that electronic components lay between the printed circuit board and the heat sink. A typical heat sink includes a plate made up of thermally conductive material, such as aluminum, and a thermal interface to transfer the heat from the electronic component to the heat sink.
Since electronic components vary in size and shape, the distance between each electronic component and the heat sink also varies. Thermal interface material, or thermal overfill, is used to maintain thermal contact between the heat sink and the various xe2x80x9cuneven heightedxe2x80x9d electronic components carried on a printed circuit board. Typically, thermal overfill is often a paste-like material which can be used to fill the spaces between the electronic components and the heat sink surface, which subsequently hardens, forming a thermally conductive layer between the electronic components and the heat sink surface. However, since the thermal overfill material has a relatively low coefficient of thermal conduction, the thicker the overfill, the higher the junction temperatures that result, with the consequent risk of component damage or failure.
In high speed applications, printed circuit boards are often provided with electronic components on the top and bottom major surfaces.
Previously, thermal overfill material could be applied to both the top and bottom surfaces of the printed circuit board. However, for electronic components on the side of the board that is not directly fixed to the inside of the heat sink surface, the tolerances included the thickness of the printed circuit board, the electronic components, the machinery, etc., with the result that the thermal overfill material had to be very thick. The temperature rise over such thickness is generally too great for heat to be adequately conducted to the heat sink surface. This results in component junction temperatures higher than allowable, increasing the risk of component damage or failure.
High speed electrical components need as short a thermal conduction path as possible between their case and their heat sink surface.
Of note, Faraday cages made of or provided with thermally conductive material may also constitute a heat sink.
This further aspect of the invention allows high power digital components to be cooled effectively through the use of thermal overfill material which is applied to both major surfaces of a plated printed circuit board having substantially the same tolerances on both surfaces.
For the purposes of determining tolerances between the electronic components and the heat sink surface, each side of the printed circuit board operates independently. It is not necessary to account for the thickness of the printed circuit board on one or the other side of the printed circuit board. A very thin overfill can be used between the electronic component case and the heat sink surface.
Advantageously, components of higher power can be used and still run at the same heat-sink surface temperature. More power can be cooled by the same amount of air flow over the assembly. Therefore, more components may be added per module.